The behavior of electromagnetic radiation is altered when it interacts with charged particles. Whether these charged particles are free, as in plasmas, nearly free, as in conducting media, or restricted, as in insulating or semi conducting media—the interaction between an electromagnetic field and charged particles will result in a change in one or more of the properties of the electromagnetic radiation. Because of this interaction, media and devices can be produced that generate, detect, amplify, transmit, reflect, steer, or otherwise control electromagnetic radiation for specific purposes.
The behavior of electromagnetic radiation interacting with a material can be predicted by knowledge of the material's electromagnetic materials parameters μ and ∈, where ∈ is the electric permittivity of the medium, and μ is the magnetic permeability of the medium. μ and  may be quantified as tensors. These parameters represent a macroscopic response averaged over the medium, the actual local response being more complicated and generally not necessary to describe the macroscopic electromagnetic behavior.
Recently, it has been shown experimentally that a so-called “metamaterial” composed of periodically positioned scattering elements, all conductors, could be interpreted as simultaneously having a negative effective permittivity and a negative effective permeability. Such a disclosure is described in detail, for instance, in Phys. Rev. Lett. 84, 4184+, by D. R. Smith et al. (2000); Applied Phys. Lett. 78, 489 by R. A. Shelby et al. (2001); and Science 292, 77 by R. A. Shelby et al. 2001. Exemplary experimental embodiments of these materials have been achieved using a composite material of wires and split ring resonators deposited on or within a dielectric such as circuit board material. A medium with simultaneously isotropic and negative μ and ∈ supports propagating solutions whose phase and group velocities are antiparallel; equivalently, such a material can be rigorously described as having a negative index of refraction. Negative permittivity and permeability materials have generated considerable interest, as they suggest the possibility of extraordinary wave propagation phenomena, including near field focusing and low reflection/refraction materials.
A recent proposal, for instance, is the “perfect lens” of Pendry disclosed in Phys. Rev. Lett. 85, 3966+ (2000). While providing many interesting and useful capabilities, however, the “perfect lens” and other proposed negative permeability/permittivity materials have some limitations for particular applications. For example, researchers have suggested that while the perfect lens is fairly robust in the far field (propagating) range, the parameter range for which the “perfect lens” can focus near fields is quite limited. It has been suggested that the lens must be thin and the losses small to have a spatial transfer function that operates significantly into the near field (evanescent) range.
The limitations of known negative permittivity and permeability materials limit their suitability for many applications, such as spatial filters. Electromagnetic spatial filters have a variety of uses, including image enhancement or information processing for spatial spectrum analysis, matched filtering radar data processing, aerial imaging, industrial quality control and biomedical applications. Traditional (non-digital, for example) spatial filtering can be accomplished by means of a region of occlusions located in the Fourier plane of a lens; by admitting or blocking electromagnetic radiation in certain spatial regions of the Fourier plane, corresponding Fourier components can be allowed or excluded from the image.